What: Swirling rumors that an acquirer might step up to buy the company caused shares in Relypsa, (NASDAQ:RLYP) to soar 67.6% higher today.

So what: The company has been the subject of M&A chatter ever since AstraZeneca plc (NYSE:AZN) forked over $2.7 billion to buy competitor ZS Pharma last year.

Relypsa markets Veltassa, a drug for the treatment of high potassium levels in the blood, or hyperkalemia. The FDA approved Veltassa last October to high expectations that stem from the fact that up to 3 million Americans with chronic kidney disease or heart failure could benefit from this therapy. Hyperkalemia can cause abnormal heart rhythms, and in some instances, can result in death.

Prior to Veltassa's approval, treatment options consisted mostly of methods that reduce potassium intake and boost potassium excretion. However, dietary restrictions can be tough to adhere to, and current methods that boost excretion lead to high patient-discontinuation rates.

The combination of those factors prompted AstraZeneca to pony up big money to buy ZS Pharma, a company with a late-stage hyperkalemia drug of its own. The FDA is set to issue its decision on ZS Pharma's hyperkalemia drug on May 26.

Now what: Investors are hoping that a similarly attractive valuation could be given by an acquirer to Relypsa, but there's a big "who knows" component to this story. That's because Veltassa's approval includes a black-box label that warns of Veltassa binding to other drugs that are taken orally.

The label recommends that patients avoid taking other drugs orally within six hours of taking Veltassa. If AstraZeneca's competing hyperkalemia drug gets approved with a less-restrictive label than Veltassa, it could become the gold standard. Given that backdrop, acquirers may exercise some caution, at least until the end of May when they'll know how the label reads.